The present invention relates to projectiles and the protection of their exterior surfaces against interior ballistic destruction or ablation. In particular, but not exclusively, it relates to fin stabilized, kinetic energy projectiles.
Fins have been used for some time in the ordnance field to stabilize projectiles in flight. After the projectile exits the gun tube, aerodynamic spin is induced by canted control surfaces on the fin blades. This spin is needed to stabilize the projectile and reduce yaw. Reducing the total yaw is extremely important in order to maximize terminal ballistic performance on target. Stabilizing the projectile gives a repeatable ballistic trajectory with a tighter dispersion pattern on target and a higher probability of hitting a target at range. For fin stabilized projectiles the loss of fin blades or severe fin ablation will destabilize aeroballistic flight and induce yaw.
Projectile designers are currently concerned with making projectiles longer to increase penetration. Kinetic energy fin stabilized penetrator designs have especially concentrated in this area. These designs have emphasized seating the projectile further back into the case. Longer projectiles have increased weights and use more propellant to give higher energy densities for the penetrator on target. In many instances, the extreme pressures and the heat transfer from the propellant bed ablates control surfaces and breaks fin blades. As these new designs seat the projectiles further back into the propellant bed, the fin blades spend a longer time in these caustic environments. The problem is not eliminated by using separate loading ammunition, as separate loading ammunition has also been known to break fin blades as well as ablate the fin blades. Gases from the burning propellant bed accelerate the propellant bags into the fin blades fracturing them.
Projectiles fired from a gun by means of a propellant charge are also subject to in-bore damage due to high propellant flash temperatures. This is particularly harmful when lightweight stabilizing tail fins of aluminum are used. Attempts have been made to protect the outer surface of such fins by anodizing, but this has not proved effective against thermal erosion or bag damage. Thermally insulating coatings of ceramic type have also been tried but these present adhesion problems and the layer thickness required tends to distort the aerodynamic characteristic of the fins. Heat absorbent coatings such as coatings containing intumescent materials are also known for their thermally protective properties but these too have poor adhesion and also undergo dimensional changes in operation which degrade the aerodynamic performance of a finned projectile.
Another example of a heat absorbent coating is that of an ablative heat shield, i.e., a sacrificial layer of material which is gradually removed by thermally induced processes, e.g., pyrolysis, melting and vaporization. Such heat shields are known for the protection of space vehicles at re-entry to the earth's atmosphere for example and are generally formed from plastics or composites having a fairly high fiber content, and often include intumescent materials. The composites are usually applied to the relevant surface either as a bonded pre-formed layer or in fluid form by trowelling or casting. Such protective layers are thick and heterogeneous, ablate unevenly and consequently would have the effect of adversely distorting the aerodynamic profile of a precise structure such as the fins of a projectile, both initially and variably during flight. Even if the coating is made relatively thin, sufficient protection is not provided from the damage caused by propellant bags in separate loading ammunition.